In road vehicles there is an increasing widespread use of electric drive in combination with traditional fuel-burning engine to provide a hybrid drive.
In a first hybrid-drive scheme, an electric machine is mechanically connected to a transmission set between the internal-combustion engine and the driving wheels. In this way, the electric machine does not require transmission elements of its own (i.e., a differential of its own and axle-shafts of its own), but shares the transmission elements of the internal-combustion engine. However, said constructional choice imposes serious problems of management of the overall dimensions, in so far as it is not always simple to find, in the proximity of the transmission of the internal-combustion engine, the space necessary to house the electric machine and the corresponding electronic power converter (which must possibly be set in the proximity of the electric machine so as to contain the length of the electrical connection wires and hence the power losses due to the Joule effect and the voltage drop and electromagnetic disturbance that occur in the electrical connection wires). In addition, if the transmission of the internal-combustion engine originally transmits the torque onto a single axle of the vehicle (i.e., in the case of only front drive or rear drive), also the addition of the electric machine does not modify said situation.
In order to overcome the drawbacks described above a second hybrid-drive scheme has been proposed, wherein a transmission of the internal-combustion engine is envisaged, which transmits the torque of the internal-combustion engine to an axle of the vehicle (front axle or rear axle), and a further transmission of the electric machine is envisaged, which is completely independent of the transmission of the internal-combustion engine and transmits the torque of the electric engine to the other axle of the vehicle (rear axle or front axle). The transmission of the electric machine is simpler than the transmission of the internal-combustion engine in so far as it does not envisage the clutch/gearchange assembly and comprises only a differential, departing from which is a pair of axle-shafts fixed with respect to the driving wheels powered by the electric machine. Said solution presents the advantage of separating the transmission of the internal-combustion engine from the transmission of the electric machine and hence of enabling a greater flexibility in the installation of the electric machine. Furthermore, said solution presents the advantage of being a highly governable engageable four-wheel-drive transmission, which supplies, when necessary, a high driving power (i.e., a high capacity for transmitting the torque to the road).
In order to eliminate the need for the differential in the transmission of the electric machine and to increase the capacity of control of the vehicle a further variant has been proposed, wherein two electric machines are provided identical to one another, each of which transmits the motion directly to a corresponding driving wheel and is preferably set coaxial to the driving wheel itself. In this variant, by driving the two electric machines in a differentiated way, it is possible to differentiate the torque applied to the two electrically powered wheels, i.e., it is possible to provide a “torque vectoring” control, which envisages application of a non-symmetrical torque (obviously only in particular conditions) to increase the propulsion and stability of the road vehicle. By way of example, when going round a bend, the internal wheel is subjected to a higher load than the external wheel and is hence able to transmit a torque to the road, which is higher than that of the external wheel. However, the use of two independent electric machines that must be set sharing one and the same axis sets big problems of overall dimensions along said axis (i.e., of overall transverse dimensions within the vehicle). In addition, the use of two independent electric machines requires the presence of two independent electronic power converters, with an increase in the overall dimensions and with a complication in the electrical connections of the electronic power converters to the electrical-energy accumulation system and in the cooling of the electronic power converters (the electronic power converters are static and hence are able to cool off by self-ventilation only to a limited extent and require an external cooling system).
The patent application No. EP1089425A2 describes an electric machine provided with: a stator equipped with a single stator winding; two shafts, which are independent of one another and are mounted so that they can turn; two rotors, which are independent of one another, are magnetically coupled to the stator, and are mounted on the shafts; and a single electronic power converter, which is connected to the stator winding for supplying the stator winding itself with a total electric current. A first rotor has a first number of pairs of poles made of non-magnetized ferromagnetic material, and a second rotor is a permanent-magnet rotor and has a second number of pairs of poles different from the first number of pairs of poles. The electronic power converter generates a total system of electric currents that circulates through the stator winding and is constituted by the union of a first, dodeca-phase, system of sinusoidal electric currents, which generates a first rotating magnetic field that turns in synchronism with the first rotor, and a second, hexa-phase, system of sinusoidal electric currents, which generates a second rotating magnetic field that turns in synchronism with the second rotor.
The electric machine described in the patent application No. EP1089425A2 necessarily has at least one rotor without permanent magnets that functions according to the principle of variable-reluctance electric machines. In fact, in the patent application No. EP1089425A2 it is excluded that both of the rotors can be equipped with permanent magnets in so far as in this situation an effect of demagnetization arises, which reduces the concentration of the magnetic flux (paragraphs [0069]0 and [0070]). However, variable-reluctance electric machines have an energy efficiency lower than electric machines with permanent-magnet rotors, and hence, as a whole, the electric machine described in the patent application No. EP1089425A2 has a relatively low energy efficiency.
In addition, the electric machine described in the patent application No. EP1089425A2 requires supply of the stator with a dodeca-phase system of electric currents, and the generation of a dodeca-phase system of electric currents entails the use of an electronic power converter that is particularly complex and costly.
Finally, the electric machine described in the patent application No. EP1089425A2 requires the stator winding to have a very small pitch, preferably equal to one, so as to have, in the rotating magnetic fields, even harmonics of significant amplitude (a necessary condition for proper control of the two rotors). However, this means that the pitch of the stator winding is significantly different from the polar pitch of the rotors, and hence, as a whole, the electric machine has a low energy efficiency.